1. Field of the Invention
The present invention relates to a wireless medium access control (MAC) protocol, and more particularly to a novel MAC protocol that may be used to avoid collisions among stations (STAs) from different overlapping basic service sets (OBSSs).
2. Description of Related Art
Basically, there are two variants of wireless local area networks (WLAN): infrastructure-based and ad hoc-type. In the former network, communication typically takes place only between wireless nodes, called stations (STAi), and the access point (AP), not directly between the wireless nodes as in the latter network. Thus, the wireless nodes can exchange data via the AP. The stations and the AP, which are within the same radio coverage, are known as a basic service set (BSS). When two adjacent basic service sets (BSSs) are located close to each other and operate at the same channel, which are referred to as overlapping BSSs, it is difficult to support the required quality-of-service (QoS) due to the possible collisions among stations from different overlapping BSSs.
The IEEE 802.11 standard specifies the medium access control (MAC) and the physical (PHY) characteristics for wireless local area networks. The MAC layer is a set of protocols which is responsible for maintaining order in the use of a shared medium. The IEEE 802.11 standard is defined in International Standard ISO/IEC 8802-111, “Information Technology—Telecommunications and information exchange area networks,” 1999 Edition, which is hereby incorporated by reference in its entirety.
The IEEE 802.11 provides a carrier sense multiple access with collision avoidance (CSMA/CA) protocol as well as polling mechanism for use as a random access protocol technique. Within such a network, if a transmitting unit wishes to transmit a packet, it may transmit a short request-to-send (RTS) packet containing information on the duration of the following packet. If the receiving unit receives the RTS, it responds with a short clear-to-send (CTS) packet. Then, the transmitting node sends its packet, and, when received successfully by the receiving node, the receiving node sends an acknowledgment (ACK) packet. However, a problem arises when there is a hidden transmitting station from the overlapping BSSs.
Currently, to address the above problem, the IEEE 802.11 specifies a Network Allocation Vector (NAV) as part of the virtual carrier sensing mechanism so that an STA, which is hidden from the transmitter of a frame, would not contend for the channel while the transmitter unit is occupying the channel. All STAs receiving a frame set their NAV for a given time period, using the duration/ID value contained in the frame, and utilize it jointly with their physical carrier sensing mechanism to determine if the medium is idle and, thus reducing the probability of collision.
Although the NAV seems to be a plausible solution, it has some drawbacks in that if two ore more overlapping BSSs are collocated and operated in the same channel, some collisions may occur. For example, FIG. 1 shows an example of WLAN composed of an access point (AP) and a number of stations (STAs) where the problem of collisions may occur among STAs from different BSSs. As shown in FIG. 1, two OBSSs are represented by the coverage area of the corresponding AP (AP1 and AP2). STA1,1 and STA1,2 belong to the BSS of AP1. If AP1 sends a beacon indicating the beginning of the Contention Free Period (CFP), STA1,1 and STA1,2 will set the NAV to the duration of the CFP. But STA2,1, which is out of the range of AP1, will not. That means that eventually STA2,1, which belongs to BSS2, can transmit frames, e.g., RTS/CTS, that can cause a collision with frames sent or received by stations, i.e., STA1,1 and STA1,2, belonging to BSS1 within its coverage area. In addition, a problem arises when STA1,1 or STA1,2, receives an RTS or a CF-END (+ACK) frame from an Overlapping BSS (from AP2). In that case, according to the current standard rules, STA1,1 and STA1,2 may incorrectly reset the NAV if no frame is received after the RTS, and will incorrectly reset the NAV upon reception of the CF-END (+ACK) frame although the frame was not transmitted from its AP.
Accordingly, there exits a need to provide an improved mechanism to avoid potential collisions from the stations in the overlapping BSS.